Passengers, please use the footbridge to cross the tracks.
Literal
Passengers [topic-は] footbridge [object-を] using tracks [object-を] cross please.
歩道橋 (pedestrian overpass/footbridge) is a compound: 歩道 (sidewalk) + 橋 (bridge). 路線を渡る (to cross the tracks/lines) uses を for the space traversed. A public announcement sentence.